This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing a bismuth oxide catalyst.
The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well known and important industrial process. Electrodeposition of primers to metal automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and the article acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the coating composition, until a coating of a desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Film forming resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through a reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives such as a catalyst to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics of the tank, the immersion time, and the like.
The resulting coated article is removed from the bath and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to form a crosslinked finish on the article. The presence of the catalyst enhances the crosslinking of the finish.
Cathodic electrocoating compositions, resin compositions, coating baths and cathodic electrodeposition processes are disclosed in Jarabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
Typical catalysts that have been used are lead and tin compounds such as dibutyl tin oxide which in some European countries have been held to be ecologically objectionable and regulations have been enacted to prevent the use of such metal catalysts. There is a need for a catalysts that will adequately catalyze the crosslinking reaction after the electrocoating composition has been electrodeposited on a substrate. Bismuth is acceptable for use in a catalyst. U.S. Pat. No. 5,554,700 to Schipfer et al issued Sep. 10, 1996 shows the use of bismuth salts of aliphatic hydroxy carboxylic acids and states that salts of relatively long chain acids such as Bi octanoate cause defects in coatings due to oil like extrusions and that inorganic bismuth compounds are difficult to disperse and are of low catalytic activity. U.S. Pat. No. 5,670,441 to Foedde et al issued Sep. 23, 1997 shows catalyst of bismuth compounds that are dissolved in an aqueous acid to make them water soluble. U.S. Pat. No. 5,330,839 to Yasuoka et al issued Jul. 19, 1994 shows the use of bismuth compounds with dialkyl tin aromatic carboxylate of an aromatic carboxylic acid. There is a need for a metal catalyst that has acceptable catalytic activity, is ecologically acceptable throughout the world, and is readily dispersed in an electrocoating composition and does not require the presence of other metal catalysts to be effective.